Vacuum tubes of television type for x-ray protection



Nov. 24, 1970 E. E. SHELDON 3,543,073

VACUUM TUBES OF TELEVISION TYPE FOR X-RAY PROTECTION Filed Jan. 25, 1968 2 Sheets-Sheet 1 FIG./ F/6./a FIG/b F/6./c

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1T1 9 554 9 8 5454 98 INVENTOR.

EDWARD EMANUEL SHELDON VACUUM TUBES OF TELEVISIONTYPE FOR X-RAY PROTECTION Filed Jan. 25, 1968 Nov. 24, 1970 E. E. SHELDON 2 Sheets-Sheet 2 FfG.6

INVENTOR. EDWARD EMANUEL SHELDON United States Patent Ofifice 3,543,073 Patented Nov. 24, 1970 3,543,073 VACUUM TUBES OF TELEVISION TYPE FOR X-RAY PROTECTION Edward Emanuel Sheldon, 30 E. 40th St., New York, N.Y. 10016 Filed Jan. 25, 1968, Ser. No. 700,607 Int. Cl. G21f 1/00, 7/02; H01j 29/28 US. Cl. 313-92 18 Claims ABSTRACT OF THE DISCLOSURE The invention relates to novel television vacuum tubes for color television. It was found that the present color television tubes emit considerable amount of X-radiation. The novel tubes described below are characterized by the X-ray absorbing face-plate which prevents the escape of such X-radiation or at least reduce the amount of said escaping X-radiation to the level which is safe for the public. These tubes have a construction in which their light transparent endwall on which the fluorescent screen is mounted has the X-ray absorbing power to accomplish this objective, which means to reduce the transmission of X-rays through said endwall to the amount smaller than 0.04 mr./ hr.

This invention relates to the image converters and image intensifiers to be used independently or in combination with television camera tubes, electron mirror tubes, storage tubes, and electron microscopes and has common subject matter with my patent application Ser. No. 392,960, filed Aug. 28, 1964, now US. Pat. 3,400,- 291, issued Sept. 3, 1968.

My invention will be useful in all situations which require the conversion of radiation from one wave-length to another wave-length of spectrum.

My invention will be useful also for intensification of the brightness of the images to be reproduced.

In addition, my invention is of great importance for improvement of resolution of images reproduced.

In addition, my invention Will make it possible to miniaturize the present image converters, and image intensifiers such as are described in my US. Pats. 2,555,423 and 2,555,424 and which were used successfully in the field of diagnostic radiology.

My invention will be better understood when taken in combination with the accompanying drawings.

In the drawings:

FIG. 1 shows the novel image intensifier.

FIG. 1a shows novel electron guide.

FIGS. 11), 1c, 1d, 1e, and 1 shows modification of the electron guide.

FIGS. 2, 2a, 3 and 4 show modifications of the image intensifier.

FIG. 4A shows an intensifier tube provided with X-ray protecting endwall.

FIG. 4B shows an intensifier tube provided with X-ray protecting shield.

FIG. 4C shows fiberoptic intensifier tube provided with X-ray protecting endwall.

FIG. 4D shows fiberoptic intensifier tube provided with X-ray protecting shield.

FIG. 5 represents a color television receiver provided with an X-ray absorbing shield.

FIG. 6 shows a modification in Which the X-ray absorbing shield is mounted on the inner surface of the face-plate.

FIG. 7 shows a modification in which the face-plate of a color television receiver tube is constructed to provide an adequate X-ray absorption.

FIG. 8 shows a black and White television receiving tube constructed to provide an adequate X-ray absorption power.

FIG. 9 shows an X-ray shield provided with elastic means for attaching to the television receiver tube.

FIG. 10 shows an X-ray shield provided with springlike members for mounting said shield on the X-ray television tube.

FIG. 11 shows an X-ray shield provided with suction cups for mounting said X-ray shield on the television tu e.

FIG. 1 shows a novel vacuum tube which comprises a photoemissive photocathode 2 such as of Cs, Ka, K with Sb, Bi or As or of a mixture of aforesaid elements, such as K-CsSb or NaK-Sb. For infra-red radiation Cs-O-Ag or CsNaKSb will be more suitable. The photocathode 2 may be deposited on the endwall of the tube 1 or on a transparent supporting plate such as a quartz, glass or mica 3 or of arsenic trisulfide. The visible or invisible radiation image of the examined object 4 is projected by the optical system 4a on the photocathode 2 and is converted into a beam of photoelectrons, having the pattern of said image. The photoelectron beam has to be focused in order to get a good reproduction of the image. In the devices of the prior art, the focusing was accomplished by electrostatic or electromagnetic lenses which are large and heavy. As a result, the standard image tubes are bulky and cannot be miniaturized. In my device, I eliminated the electrostatic or electromagnetic lenses which made it possible to make a miniature device. The problem of focusing the electron beam without the use of electron-optical devices, was solved by the use of a novel mechanical device such as the apertured guide 5. The guide 5 comprises a plurality of tunnels 6, each tunnel is of a microscopical diameter and extends through the whole length of the guide. Each of the tunnels must be insulated well from the adjacent ones. It was found that there are various ways to construct such guide. In one preferred embodiment the guide 5A may be constructed of a plurality of hollow tubes 15 of glass or of plastic, having their both ends open and being of ten microns diameter or less, and held together by silicone or other temperature stable plastics or by fusing them together by heating, see FIG. 1a. For a good resolution of the image, I use BB -250,000 of such tubes stacked together in one square inch area. In some cases each of tubes 15 is coated on inside walls with a conducting layer, such as of aluminum 7a or semi-conducting layer 7c, which is connected to an outside source of electrical potential.

The tubes 15 may be also held in position at their ends only either by fusing them at the ends only, by heat, or by gluing them together with silicone or other plastic material compatible with vacuum or mechanically, for example, by threading their ends only into a mesh screen mounted rigidly in the tube.

In cases in which resolution of images is not important, the guide *5 may be constructed of a number of apertured glass plates combined in one unit as was described above for tubes 15. In the preferred embodiment of invention the tubes 15 of glass or plastic may be coated on their outside walls with a conducting material 7a or semi-conducting material 70 and next with the insulating material such as of fluorides, glass, plastic, MgO, or silicon oxide 50, extending along the entire length of said tubes and around their entire circumference. Next the inner glass or plastic wall of the tubes 15 is leached out to make the conducting 7a or semi-conducting layer or resistive 7c face the lumen of the tunnels 6. In this construction an insulating coating 50 is of material resistant to the leaching agent and it will serve as a support for other layers. The material for uniting the tubes 15 should be resistant to temperature necessary for vacuum processing. Plastic materials such as fiuoro-carbons, polyethylenes such as fluoroethylenes or silicon compounds such as silicates are useful.

If the tubes are united by heating them, the outer Walls of the tubes may be clad before the fusion with a glass or other material which is resistant to the leaching agent and which melts easier than the layer 50. In some cases the dielectric layer 50 may serve for this purpose as well.

In some cases, the first coating to be applied to the walls of the tubes 15 may be of a secondary electron emissive material as shown in FIG. 1d which may be of semi-conducting type such as CsSb, of insulating type such as of fluorides, MgO, or alkali halides such as KCl or of aluminum oxide, or of conducting type such as Be, Ni, Cu, or of a mixture thereof. In some cases layer 50 and 7a or 70 should be able to tolerate temperature of 600 C. The dielectric layer 50 as was explained above serves as a support for all other layers and extends along the entire length of the tunnels.

The secondary electron emissive layer 20 should preferably extend along the entire length of the tubes and cover the inside lumen of tunnels 6 on all sides.

In some cases the coating 20 may be also applied to the inside walls of the tubes 15, after they have been coated with the conducting and insulating layers and after they were leached as was described above, but the results are inferior than in the method described above.

It is also possible to coat the inside walls of the tubes 15 with a conducting layer and with a secondary electron emissive layer 20 by evaporation or electrolytically. In such case the tubes 15 do not require any leaching at all. The results however are inferior to the method described above because the secondary electron emissive coating is not uniform. In my preferred construction the deposition of the secondary electron emissive material is done on the external surface of the walls of said tubes which makes it practical to produce a homogenous and uniform deposition of the secondary electron emissive material. As was explained above the subsequent leaching of the glass makes the secondary electron emissive material face the lumen of the tunnels 6b.

Another preferable method of building the guide 5 is to use a fiber plate which consists of plurality of fibers of 5 to 10 microns diameter made of glass or plastics.

The fibers are coated with a dielectric material 50 such as suitable glass, plastic, fluorides, silicon oxide or other silicon compounds, as shown in FIG. 1]. In some cases the fibers and thin coating sould be able to tolerate temperature of '600 C.

The material for uniting the fibers should be resistant to leaching agent used for the glass and also resistant necessary for vacuum processing. Among plastic materials fluoro-carbons, polyethylenes such as fiuoroethylenes or silicon compounds are the best. All these fibers are glued chemically or are fused together by heating. Such a fiber-plate is now subjected to a leaching process in which the glass or plastic fibers are etched out and dissolved by a suitable chemical. The leaching agent does not attach however, the coating of fibers. We will obtain therefore, after the leaching is completed, a guide 5F having as many tunnels 6 as there were original fibers in the plate. The fiber-plates can be constructed of fibers having only six microns in diameter. Therefore the tunnels 6 will have a diameter of approximately 6 microns. If it is important to have the tunnels of a uniform diameter, the fiber plate should be made of fibers which have a coating of glass or plastic which does not deform during the heating fusion. In some cases it is preferable to have an electrically conducting coating on the inside Walls of tunnels 6. In such case, a layer of Al, Pd, Au or Ag may be deposited on the inside walls of the tunnels 6 either by evaporation or electrolytically. A preferred method of providing a conductin 7a or semi-conducting or resistive 7c coating inside of tunnels 6 is to use the fiber plate in which the fibers before combining them in one unit are clad with a metallic coating or in which the dielectric coating such as of glass or plastic comprises a metal. In such case an additional insulating layer '50 which may be of a glass, plastic, fluorides or silicon oxide or silicates should be deposited outside of the metallic layer to provide a good electrical insulation of tunnels 6 from each other. It should be understood that tunnels 6 and all their modifications have the length a few times, which means at least two times, larger than the diameter of their apertures 42.

Also fiber-optic mosaics may be used for construction of the electron guide 5 such mosaic can be made of a plurality of fibers, having a core of one kind of glass and a coating of another type of glass. All these fibers are fused together by heating. Such a fiber-optic plate is now subjected to a leaching process in which the core of the fibers is etched out and dissolved by a suitable chemical. The leaching agent does not attack however, the coating of fibers. We will obtain therefore, after the leaching is completed, a plate having as many tunnels as there were original fibers in the fiber-optic mosaic. It should be understood that these glass fibers may be also provided with a coating of secondary electron emissive material 20 and of the conducting material 7 before being coated with another type of glass. Therefore after the core of said fibers is leached out the secondary electron emissive layer will face the lumen of tunnels '6.

I found that the channels made of the metal tubes in the prior art could not give a good resolution of the images because the metal tubes could not be made of diameter smaller than 0.50 mm. and could not be reproduced uniformly. In my device glass or plastic tubes are used which can be produced of diameter of 0.01 mm. and which can be produced with a great degree of uniformity in great numbers. My device will need 200,- 000 tubes or more.

It should be understood that the word glass in the specification and in the claims embraces all kind of glasses and synthetic plastic materials as well.

Another electron guide is shown in FIG. 1c. The vacuum tube 1A has a source of electrons such as photocathode 2 or an electron gun 40 and a novel electron guide 5C.

The guide 50 comprises in vacuum tube 1A a plurality of perforated members 60 such as plates or meshes of dielectric material, such as glass or plastic and a plurality of electrically conducting perforated members 61 such as plates or meshes of steel, nickel or copper. The dielectric members 60 and conducting plates or meshes 61 are stacked together and glued together or fused in an alternating pattern. In this Way plural channel 6a are produced which have walls of alternating strips of dielectric material and of a conducting material. All electrically conducting members 61 may be connected to an outside source of potential.

An improved method of producing apertured plates or meshes is to use a fine focused electron beam for perforating continuous sheets of suitable materials. This method is used for electrically conducting materials such as nickel, copper beryllium and for dielectric materials such as plastics, fluorides or glass as Well.

In some cases it is advantageous to intensify electron beam by a secondary electron multiplication. This is accomplished in my invention by coating the perforated apertured conducting members 61 0f the guide 5D in vacuum tube 1b with a secondary electron emissive material 20a such as calcium fluoride, alkali halides, such as KCl, alucminurn oxide, CsSb, and N1 or Be, of the thickness of 50 to 250 angstrons as shown in FIG. 1b. This coating 20a may be deposited by evaporation or by electrolytic process, and is deposited before thte members 60 and 61 are combined together in one unit, their apertures being aligned and forming thereby elongated tunnels 6a having the length larger than diameter of said apertures. It should be understood that the various arrangements of dielectric members 60 and of conducting members 61 coated with layer 20a come within the scope of my invention. For example, I may use a few dielectric members 60 for each conducting member. The conducting members 61 coated with the layer 2011 are connected to an external source of the electrical potential. Each member 61 is provided with a potential/kv. one higher than the preceding one. In the vacuum tubes of the prior art the emitted secondary electrons had to be focused by means of bulky magnetic devices to prevent loss of resolution. In my device, all electron-optical focusing devices can be eliminated and still a better resolution is obtained than in the prior art. The secondary electrons must travel through the tunnels 6a and are restrained to the size of such tunnels. The tunnels 6a or 6 should preferably be in some cases at an angle to the photocathode 2. In some cases the apertures 42 of tunnels 6 or 6a should have a bevelled shape.

It was found however that the perforated plates of meshes whether of conducting type or of dielectric type cannot give as good resolution, as the electron guides made out of hollow tubes or of fibers which were described above. It was also found that conducting mesh screens covered with insulation and stacked together do not make tunnels of uniform diameter and shape as it is required for the best resolution of the images as it is impossible to bring plurality of such screens into a perfect registry with each other as it was successfully done in electron guides using hollow tubes or leached out fiber-plates.

My novel imaging devices may use all embodiment of the electron guides described above. The novel image tube 1 shown in FIG. 1, as described above, has the photocathode 2 on the support 3, electron guide 5 and an image reproducing screen 8. The image reproducing screen 8 comprises luminescent or electroluminescent material such as ZnSCdS, ZnSAg or zinc silicate and is covered on one side with an electron transparent, light reflecting layer 9 such as aluminum. The layer 9 prevents the light emitted by the screen 8 to scatter back to the photocathode 2. The image of the examined area 4 is projected by the lens 5 on the photocathode 2 and is converted into a beam of photoelectrons having the pattern of said image. The photo-electron beam is accelerated by the electrical fields 39, enters the guide 5 through the apertures 42 and. is focused by said guide onto luminescent screen '8. It leaves the guide through the apertures 42a, is accelerated again by the fields 39, strikes the screen 8 and reproduces a visible image therein. This novel image tube does not require any electron-optical focusing devices for good resolution of the image.

I found that the closer the guide 5 is to the photocathode 2, the better is the resolution of the image. In particular, a distance of a small fraction of one millimeter will give the best results, the distance of a few millimeters will give a much worse resolution. The vacuum tube 1 shown in FIG. 1 must be provided with a unidirection electrical potential for acceleration of photoelectrons from the photocathode to the guide 5, and from the guide 5 to the image reproducing screen 8. The accelerating potential may be applied to the conducting cylinders which transmit electrons or coating 39 on the inside of the tube envelope or to the conducting layer 7 such as of aluminum. The higher the accelerating potential is, the brighter the reproduced image will be in the screen 8. There is, however a limit to the strength of the accelerating potential which is set by the dielectric strength of the tube. The use of guide 5 allows the potential to be spread between the photocathode 2 and screen 8 over a longer distance and without loss of resolution. Therefore it will be possible now to use, in the tube 1, a much higher potential than it would be feasible without said guide 5. The conducting layer 7 may be 50-100 A. thin so it will be completely transparent to the photoelectrons emitted by the photocathode 2. The conducting layer 7 or semiconducting layer is connected to an outside source of potential and may be preferably in contact with the conducting or semi-conducting coating on inner Walls of tunnels 6. The layer 7 may be continuous. In some cases, a perforated metallic layer 7b will be better. The perforations in the layer 7 corersponding to the apertures 42 of the tunnels 6, may be made by blowing a strong current of air through the tunnels 6. Another method of producing the apertured conducting member is to use a perforated plate or mesh screen of conducting material such as 43 described below.

The length of the tunnels 6 in the guide 5 must be longer than the diameter of the apertures 42 of said tunnels. The actual length will vary according to the application of my guide and the type of vacuum tube. However the tunnels of the guide should be at least a few times longer than the diameter of the apertures. The longer is the guide 5, the greater difference of potential can be applied to both sides of said guide. The greater is the potential difference, the more acceleration of the electrons can be achieved. This brings about a greater image intensification, which was one of the purposes of my invention. The acceleration potentials may be supplied from an external source of potential connected to the layer 7 or 43 or to separate grids which transmit electrons and are disposed on both sides of the guide 5, or 10 to conductive rings 39 mounted on the walls of the vacuum tube. In the devices of the prior art, it was impossible to provide a large potential difference, because the separation of the fluorescent screen 8 from the photoelectric screen 2 could not be longer than 0.25-05 cm. exceeding this distance caused a prohibitive loss of resolution of the image. In my device, in spite of the elimination of the focusing electron-optical lenses or fields, I can provide separation of the photocathode 2 and of the fluorescent screen 8 of any desired length without a loss of resolution of the image. I found that for the best resolution in this embodiment of invention the walls of the tunnels 6 facing the lumen of said tunnels should be free from a photoelectric material or from a secondary electron emissive material.

The electron beam from the photocathode 2 carrying the image is therefore guided by the electron guide 5 to the image reproducing screen -8. It is accelerated to impinge on said screen 8 with a suific-ient velocity to produce therein a visible image of increased brightness.

The tunnels 6 may be uniform in their diameter through the whole length of the guide 5. The tunnels 6 may have also a divergent form, in which the exit apertures are larger than the entrance apertures. In such case the electron beam will be enlarged upon its exit from the guide. The tunnels 6 may be also of a convergent form in which the exit apertures are smaller then the entrance. In this case the electron beam will be demagnified on its exit from the guide.

The separation of guide 5 from the photocathode 2 will cause some photoelectrons to strike the solid parts of guide 5, instead of entering the apertures 42 in the guide. In this way, a space charge may be produced on solid parts of guide 5, which may interfere with the photoelectron image. I found that development of the space charge is the cause of failure of such devices. The conducting layer 7 will prevent this from happening as the charges will be able to leak away through layer 7. In some cases, it is preferable to mount guide 5 in contact with the photocathode 2 or the photocathode may be deposited directly on the end-face of guide 5 instead of on the endwall of the tube or on a supporting: member 3, as is shown in FIG. 2. In this construction the conducting layer should be a perforated layer 7b or a perforated member 43. The discontinuous electrically conducting layer 7b may be also made by evaporation and will have -90% transmission for electrons. In some cases it is preferable to use an electrically conducting member 43 in the form of a metallic wide mesh screen or perforated plate of metallic material or of a perforated member coated with an electrically conducting material such as tin oxide. The member 43 is mounted on the end-face of the guide in such a manner that openings of the screen or plate 43 coincide with one or with a few apertures 42 of the guide 5. The screen or mesh 43 is connected to an outside source of electrical potential in the same manner as layer 711. In this construction I found that a problem arises because of the chemical interaction between the photoemissive material of photocathode 2 and the materials of guide 5. It is important, therefore to select materials which do not poison the photocathode. Lanthanum glass is chemically compatible. Still a protecting separating layer 2a of a light transparent material such as of calcium fluoride, MgO, or of silicon monoxide may be needed. The layer 2a should be preferably perforated and have a transmission for photoelectrons of 80%90%. The apertures of the layer 2a must coincide with the apertures 42 of the guide. The layer 2a may be prepared by deposition on the top of the layer 43 of a continuous layer first and next by rupturing said layer with a strong current of air blown through tunnels 6, so that only the parts overlaying the solid portions of the guide 5 will remain in position.

Also, the phosphor screen 8 may be deposited directly on the end-face of guide 5. This construction facilitates markedly the construction of tube 1, as guide 5 with the image reproducing screen 8, and in some cases also with the photocathode 2 may be prepared outside of vacuum tube 1, and then introduced into tube 1a in one unit, and mounted therein.

In some cases, either only the photocathode 2 or only the image screen 8 are in contact with the guide 5. In case the screen 8 is separated from the guide 5, the separation, for the best results, should be preferably a fraction of one millimeter.

In some cases it is preferable to prevent the electrons which travel through the tunnels 6 or 6a in the guide 5 from striking the walls of said tunnels. This can be accomplished by providing the walls of said tunnels which face the lumen with a conducting or semi-conducting coating 7a as shown in FIG. 2a. The conducting coating may be of aluminum or chromium. The semi-conducting coating may be of tin oxide or of titanium oxide. The coating 7a may be connected to the perforated conducting member 43 or to layer 7 which again may be connected to an outside source of electrical potential. As all tunnels 6 are in contact with the layer 7 or with member 43, walls of said tunnels will have a potential which will repel electrons travelling through said tunnels.

In some cases, the second perforated member 43 or 7 mounted on the opposite end of the guide 5, may be discontinuous from the coating 7a by terminating said coating 7a before reaching one end-face of the guide 5. In this construction, the second member 43 may be connected to the external source of electrical potential to provide acceleration for electrons.

In the embodiment of invention, shown in FIGS. 1 and 2, and 2a, the tunnels 6 of the guide 5 run normally to the photocathode 2 and are straight from the beginning to their end to prevent photoelectrons from striking the inside walls of the tunnels.

It will be understood that my device may use a plurality of electron guides 5. In such case electron accelerating means such as grids, rings, cylinders or meshes connected to a suitable source of potential may be interposed between the electron guides.

The semi-conducting coating or resistive 7c in some cases is preferable to conducting coating because it allows to establish potential gradient along the length of the tunnels 6. This potential gradient will cause acceleration of electrons into direction of the exit apertures 43a if it is connected to a suitable source of electrical potential.

In many cases it is advantageous to intensify electron beam by a secondary electron multiplication eg. by coating the inner walls of the tunnels 6a with a secondary electron emissive material 20 such as CsSb, Ni, Be, calcium fluoride, alkali halides such as KCl or aluminum oxide or others. This coating 20 may be deposited by evaporation into tunnels 6, but the deposition is not uniform for the best results. In a preferable modification of this invention the secondary electron emissive coating 20 for the inner walls of the tunnels 6 may be provided by the methods which were described above. The glass or plastic fibers 38 before being fused or glued into a fiber-plate are coated with a secondary electron emissive material 20, such as was described above. On the top of said coating 20 an electrically conducting coating 35 is applied, such as of chromium, aluminum or nickel. On the top of the conducting coating 35, a dielectric coating 36 such as of glass, plastic or of fluorides is applied, which will serve to fuse all fibers into one fiber-plate as shown in FIG. le. It should be understood that the coatings 20, 35 and 36 must be of material resistant to the action of the chemicals used for etching out the fibers. After the fiberplate is prepared, and the fibers are leached out, we obtain the tunnels which have the following layers. The layer facing the lumen of said tunnels is the secondary electron emissive layer 20, the next layer is the electrically conducting layer 35, the next layer is the insulating layer 36. The conducting layer 35 may be connected to the source of suitable potential for the best secondary electron emis- SlOIl.

In some cases, instead of conducting layer on the inside walls of the tunnels 6 it is better to have a layer of semiconducting material 7c such as of tin oxide, titanium oxide, or zinc fluoride. It should be understood that the use of semi-conducting coating instead of a conducting coating applies to all modifications. In some cases an electrically resistive evaporated layer 70 may be used instead of a semi-conducting layer 70. The resistive layer in a modification of my invention instead of being a base for the electron emissive layer 20, may replace it and serve to provide electron multiplication. In this construction the tunnels 6 should be at an angle to the photocathode or the photocathode at an angle to the tunnels.

The operation of the modification of my invention using secondary electron emissive layer 20 is shown in FIG. 4. The photoelectrons entering the aperture 42 are directed into said apertures at an angle so that they will impinge on the walls of said tunnels 6 coated with layer 20. In this construction apertures 42 are slanted at an angle of 45- and tunnels 6b in the guide 5 are straight or at an angle in relation to the photocathode 2. In some cases in order to provide the obliquity for the entering photoelectrons; instead of the tunnels, the photocathode 2 may be mounted at the angle. In such a case the tunnels will be normal in relation to the end-wall of the tube. The angle at which photoelectrons enter will depend on the size of apertures and their spacing from the photocathode. The photoelectrons must have only a few hundred volt velocity to produce secondary electron emission greater than unity from the layer 20. The low accelerating voltage in front of the photocathode 2 creates the problem of resolution. As was explained above, my device is characterized by the absence of electron-optical focusing means. The photoelectrons leaving the photocathodes have a range of velocities 0.5 volts-10 volts according to the wave-length of radiation used. The use of 300 to 1,000 volt accelerating potential requires a much closer spacing of the photocathode 2 to the end-face of the guide 5 than devices in which the accelerating potential is a few thousand volts. It was also found that the use of the low accelerating voltage required that the conducting layer 7 be of perforated type such as layer 7b or a perforated member 43 because electrons of a low velocity will not be able to penetrate continuous layer 7.

The inside walls of the tunnel 6 should have a progressively higher potential along their length in order to cause repeated impingement of secondary electrons on the layer 20 while they are traveling to the exit apertures. It was found that the best way to provide progressively higher potential for the walls of the tunnels 6 is to divide the electron guide into plural segments and to interpose between said segments apertured electrically conducting members 43 or apertured layer 7b or conducting rings which can be connected to various electrical potentials required. The conducting layer 7a or semiconducting layer 7c which are on each tunnel are connected to said apertured electrically conducting members. This construction aflfords a simple and practical solution of supplying progressively higher potential to all tunnels 6 in spite of the fact that we may use 200,000 tunnels or more in one electron guide 5.

In some cases additional electron accelerating means such as cylinders or rings are provided between the exit apertures of tunnels 6 and their modifications and the image reproducing fluorescent screen 8. In such cases focusing means of magnetic or electrostatic type are provided to keep the accelerated electron beam in focus. The accelerating potentials may be used up to 3540 kv. This construction will result in a marked intensification of the reproduced image in the screen 8.

It was found that all image intensifiers described above and which operate with accelerating potentials above 25 kv. suffer from discoloration of the glass of their endwall 8a or 8c. The reason for said discoloration was found to be due to multi-crystal granular and discontinuous construction of the phosphor screen 8. This allows some electrons of high velocity to pass between the crystals or particles of fluorescent material and impinge on the glass endwall of the tube. This problem was solved by the use of a very thin layer 55 which is transparent to fluorescent light and Which is absorbing for electrons. The layer 55 is mounted on the internal surface on the endwall 8a Or 80. This protective layer 55 should be of the thickness not smaller than 1 micron and does not have to exceed the thickness of microns. The layer 55 may be made by evaporation or by deposition from a solution. The main requirement for this layer is that it should have a continuous construction over its entire surface which means that it cannot be in the form of a discontinuous aggregation of particles, as it is in the light transparent layers made of metals. The layer 55 must be also chemically compatible with the fluorescent material used in screen 8 and must be well adherent to the glass of endwall 8a or 80 and to the fluorescent material of screen 8 to prevent peeling off of said fluorescent screen 8. Materials such as compounds of silicon in the form of silicon oxide, aluminum oxide, plastic silicones or of an evaporated zinc sulphide, as distinguished from a standard settled type, of zinc sulfide were found to be suitable for the layer 55. An important feature of the layer 55 is that it should be of material which has the index of refraction smaller than the fluorescent material of the screen 8 and higher than the index of refraction of the endwall 8a or 80 of the vacuum tube. Such construction will eliminate the reflections of the light from the layer 55 and endwall which degrade the contrast of images produced.

It was found however that the screen 8 when used with kv. potential or higher besides producing fluorescent image emits also undesirable X-rays which escape in large amount through the endwall of said tubes. The emitted X-rays escape from the tube 1 or its modifications in large quantities because the endwall of said tubes is made of only 1 to 2 mm. thickness. The transmitted X-rays have wave lengths longer than 0.35 A. The standard glasses used for vacuum tubes such as Pyrex absorb only about 50% of such X-rays for one millimeter of the thickness of the glass. As a result a dangerous amout of X-radiation impinges on the observer whose eyes are usually in a close proximity to the fluorescent screen 8. The solution of this problem was found in the use of the endwall 8a of the tube 1 or its modifications, as shown in FIG. 4A, which has the thickness not less than 7 mm. and preferably between 7 mm. and 1.3 mm. when using standard glasses for vacuum tubes. This thickness may be reduced by making the endwall 8a of special glass which contain oxides of heavy metals; especially barium oxide, lead oxide and cerium oxide were found to be very efiicient for this purpose. In another modification of this invention shown in FIG. 4B the additional X-ray protection for electron multiplying tubes 1 or their modifications and which have the thickness only of l to 2 mm. of glass was provided by the use of an additional X-rays absorbing shield 8b mounted on standard endwall 8c of said tubes. The X-ray absorbing shield 8b may be made of glass or transparent plastic material such as acrylates, polyamides or terephthalates. The thickness of such shields should be not less than 5 mm. and preferably between 5 mm. and 8 mm. This thickness may be reduced by incorporating into said shield oxides of metals such as barium oxide, lead oxide or cerium oxide.

The shield 812 should be mounted on the external surface of the endwall 8b of the tube 1 or its modications and not on the internal surface of said endwall. It was found that the shield 81) mounted on the internal surface of endwall causes increased reflections of fluorescent light from the endwall 8c and damages the contrast of reproduced images.

The shield 8b which is of a light transparent X-ray absorbing glass or plastic should extend beyond the borders of the endwall 8c of said tubes in. all directions in order to intercept divergent X-rays.

It should be understood that shield 812 may be also mounted on the improved X-rays absorbing endwall 8a. In such case the X-ray absorbing power of the shield 8b may be accordingly reduced.

In some cases besides using the improved X-ray absorbing endwall 8a alone or with a shield 8b, the X-ray protection may be obtained by the use of a casing for the intensifier which has suflicient X-ray absorbing power as was explained above.

It should be understood that the necessity for the X-ray protection applies not only to the above described electron multiplying tubes but also to fiberoptic image intensifiers shown in FIGS. 4C and 4D which operate without any electron multiplying means. Such fiberoptic image intensifiers are usually combined in a double or triple tandem by coupling said tubes by means of their fiberoptic endwalls 54 and 54a. The last endwall of the tandem combination does not have any fiberoptic construction, is of the standard vacuum tube glass and has the thickness limited to 12 mm. only. As was explained above, such endwall is not sufficient for the X-ray protection; especially in view of the fact that such fiberoptic devices operate at 3040 kv.

It should be understood that all X-ray protective means described above apply as well to fiberoptic tubes, which are shown in FIGS. 4C and 4D.

This invention relates also to novel television receiver tubes which are constructed to provide protection from the X-radiation emitted by said television receivers. It was found that modern television tubes which operate at the potential of 16 kv. to 28 kv. produce X-rays at the impact of the electrons of the scanning electron beam on the television image reproducing screen or on side walls adjacent to said screen. The emitted X-rays are very soft but they escape through the endwall of the television tube which means the face-plate of the television receiver tube, and represent therefore a health hazard. The present television sets are provided with endwalls or faceplates of glass which absorb a large percentage of such X-rays. It was found however that this protection is not adequate and that it should be increased. The need for additional protection from X-rays is due to the fact that children watch television programs from a short distance from the television screen and receive therefore an X-ray exposure many times greater than adults who watch television from a conventional distance. In addition it is an established fact that growing issues in childhood are much more sensitive to the effect of X-radiation than the adult tissues. It is also an established fact that X-ray effects are cumulative and may manifest themselves only 35 to 50 years from now.

FIG. shows a color television receiver tube 51 which may be of all-glass or metal-glass type. The television receiver 51 is provided with an electron gun 62, which may be in the form of a single gun or may consist of 3 different electron guns as it is well known in the television art. The electron gun produces an electron beam 62a of standard current which in color television is about one milliampere and which serves to scan the television image producing screen 64. Between the electron gun 62 and the image screen 64 is mounted shadow mask 63 which serves to direct the scanning electron beam 62a to proper area of the image reproducing screen provided with a fluorescent screen 64. The accelerating means 52 serve to provide accelerating potential for the scanning electron beam 62a such as 27 kv. to 40 kv. for color receiving tubes. The electron beam focusing means and deflecting means are not shown, as they are well known in the art. The image reproducing screen 64 comprises fluorescent materials such as sulphides, silicates or other phosphors and may be of 2-color or 3-color type. The image reproducing screen 64 is provided with an electron transparent light reflecting layer 65 such as of aluminum of the thickness 10005000 A. It should be understood that the present invention applies to all types and varieties of color or black and white television receivers, which use the voltage exceeding 24 kv. but not exceeding the voltage of 40 kv. The special field of application of the invention are color television tubes operating at the potential of 27 kv. and in actual practice often at 30 kv./or at higher potential, but not exceeding the voltage of 40 kv.

The impingement of the scanning electron beam 62a on the screen 64 produces in addition to the visible television image also X-rays which escape from the receiver tube 51 through its face-plate 66 to the outside. The faceplate 66 of the television receiver tube 51 (called also kinescope) is of glass, or other light transparent vacuumproof material. In the present color television tubes operating at 27 kv. or at a higher potential the face-plate is of the thickness of about 1 cm. This face-plate should reduce the amount of X-radiation escaping from the tube to the dose of 0.5 milliroentgen per hour at the distance of 5 cm. from the face-plate of said tube, as it is required by the National Council of Radiation Protection. It is believed however that this X-ray dose should be reduced at least to 0.1 mr./hour. In some cases such reduction must be at least by a factor of 12 to 20 or preferably more. The need for additional protection from X-rays is due to the fact that children watch television programs from a short distance from the television screen and receive therefore an X-ray exposure many times greater than adults who watch television from a conventional distance. In addition it is an established fact that growing tissues in childhood are much more sensitive to the effect of X-radiation than the adult tissues. It is also an established fact that X-ray effects are cumulative and may manifest themselves only 30 years from now.

It is another established fact that the human lens is subject by the natural process of aging to a cataract formation. It is another established fact that the X-ray exposures cause cataracts formation. The danger exists therefore that even small but continuous exposures to X-radiation of children eyes will cause premature aging of the lens and premature cataract formation.

The only known safe dose of X-radiation when it comes to an X-ray exposure for long periods of time is so called background radiation dose. This is the amount of X- rays to which human beings are always exposed from the external and internal natural environment. This background" radiation dose amounts to milliroentgen or 100 millirads per year only. It should be understood however that this background dose cannot be applied to determine the safety of the X-ray exposure for the eye, as it was mistakenly believed. It was proved that the use of background radiation dose as a safety guide is not applicable in populations of civilized countries. One dental X-ray examination delivers 100 to 1000 times more X- radiation to the eye than the whole year of the background radiation. It was found that the eye receives in one dental X-ray examination 1.2 r. to 10 r. (1200 millirads-l 0.000 millirads), whereas the background radiation dose is only about 100 millirads per year.

In view thereof of the safety limits of -background radiation dose are already exceeded by the factor of 10 to 100. It follows that the comparison of the X-ray exposure due to television receivers with the background radiation cannot serve any useful purpose to determine the safety. The danger exists that any additional X-ray exposure may precipitate the damaging effects of X-rays to the eye.

It is therefore the purpose of this invention to improve the safety of television receivers by a factor of at least 12 to 20 or more. This is accomplished by providing an X-ray absorbing shield 67 attached to the face-plate 66. The X-ray shield 67 may be cemented to the face-plate by clear epoxy cement or other plastics such as silicones or polyesters. The X-ray shield 67 may be also united with the endface 66 by other chemical means such as solder glass e.g. Corning solder glass or by any mechanical means. It should be understood that all means of mounting or attaching the X-ray safety shield 67 come within the scope of this invention. The X-ray shield 67 may be made from a light transparent glass or a suitable light transparent plastic material such as acrylates or terephthalates or polyesters and should be preferably of the shape corresponding to the configuration of the external surface of the face-plate 66 or 78 so that it may be brought into the optical contact with said face-plate. It should be understood that the shape of the X-ray shield 67 may be in some cases different from the face-plate 66 and in such case an optical cement may be used to fill-in the differences in the respective surfaces of the face-plate and of the X-ray shield. The X-ray shield 67 may be of curved shape or of flat shape and should preferably extend beyond the area of the face-plate 66 or 78 to provide additional protection from the stray X-ray coming either from the edges of the screen or from the sidewalls of the cone 51a. In another modification the X-ray shield 67 may be bent to follow the sidewalls of the cone 51a. The absorbing power of the X-ray shield 67 will depend on the X-ray absorption by the face-plate 66. The X-ray shield 67 is constructed from light transparent glass or plastic which is provided with oxides of heavy metals such as lead oxide, barium oxide, cerium oxide or titanium oxide, which absorb well X-rays. The thickness of the X-ray shield will vary considerably according to the type of the X-ray absorbing material used and its percentage and may vary between 2.5 mm. and 7.5 mm. The best results were obtained using lead oxide. Whatever the thickness is selected, the combined X-ray absorbing power of the face-plate 66 and of the X-ray shield 67 must be such as to reduce the amount of X-rays escaping through them to the rate of not more than 0.1 milliroentgen per hour. In many cases, especially for protection of children it is obligatory to reduce the escaping X-rays to the rate not exceeding 0.040 milliroentgen per hour; all aforesaid X-ray intensities being measured at 5 cm. distance from the face-plate 66 or 78 or from X-ray shield 67 or 76.

This invention applies especially to the color television receiver tubes in which the peak energy wavelength of the emitted X-rays from the fluorescent screen 64 in said receivers is shorter than 0.5 A. but does not exceed 0.35 A., which means that this wave-length is not shorter than 0.35 A.

should be of the thickness not smaller than 1 micron and does not have to exceed the thickness of 20 microns. The layer 55 may be made by evaporation or by deposition from a solution. The main requirement for this layer is that it should have a continuous construction over its entire surface which means that it cannot be in the form of a discontinuous aggregation of particles, as it is in the light transparent layers made of metals. The layer 55 must be also chemically compatible with the fluorescent material use in screen 64 and must be well adherent to the glass of endwall 66 and 68 and to the fluorescent material of screen 64 to prevent peeling off of said fluorescent screen 64. Materials such as compounds of silicon in the form of silicon oxide, aluminum oxide or plastic silicones or an evaporated zinc sulphide, as distinguished from a standard settled type construction of zinc sulfide, were found to be suitable for the layer 55. An important feature of the layer 55 is that it should be of material which has the index of refraction smaller than the fluorescent material of the screen 64 and higher than the index of refraction of the endwall 66 or 68 of the vacuum tube. Such construction will eliminate the reflections of the fluorescent light from the layer 55 and endwall 66 or 68 which degrade the contrast of images produced.

It was found that the presence of the X-ray shield 67 or 76 produces undesirable halation effects which reduce the contrast of the television images. One solu tion of this problem was found to be introduction of light absorbing materials into the X-ray shield and which serves to reduce multiple internal reflections of light in said X-ray shield. Such filtering material disposed in the shield improves markedly the contrast of television images because the light rays which are subject to multiple reflection are more absorbed than the light rays which are directly transmitted from the fluorescent screen to the outside through said X-ray shield. It was found that the use of light filtering materials incorporated into the X-ray shield is technically simpler and less expensive than the use of said filtering material in the relatively thick face-plate 66, 68 and 7 8. Unexpectedly it was found that the use of the light filtering material is the X-ray shield 67 or 67a or 76 may remove the need for the light filtering in the face-plate itself. This improvement resulted not only into lower production cost but also offered a better light transmission which is of paramount importance in color television receivers which have to compensate their low image brightness with high voltages.

If the X-ray shield 67 or 76 is attached to the faceplate 68, 66 or 78 which has already the light filtering material, the X-ray shield may be still preferably provided with its own light filtering material. This light filter ing was accomplished by introducing into the X-ray shield 67, 67a or 76 and made of plastic or glass, small amounts of metals such as iron, manganese and cobalt. The addition of such metals produces formation in the glass of compounds of different valence values which absorb selectively various wave-lengths of the light. If the amount of the red light, green light and blue light will be absorbed proportionately to their content in the white light, the transmitted light will be attenuated but will not exhibit any colors. For example, iron will absorb red wavelength, and cobalt will absorb green wave-length. As a result the transmitted light will have a grayish colorless appearance.

In some cases the X-ray shield 67, 67a, 76 or its modifications may be made of borosilicate type of glass instead of a soft glass such as soda-lime glass. This material may be converted into a colorless filter by means of adding to such glass the iron only. In some cases the X-ray shield 67, 67a or 76 should have light opaque side- Walls. This construction of the X-ray light shield traps the inner reflections of light in the shield and improves the contrast of the television images.

In addition it was found that it is preferable to render 16 the front which means the external surface of the X-ray shield 67 or 76 to be light diffusing such as by frosting lightly said front surface and following it with hydrofluoric acid treatment or by etching; said front surface. This construction improves markedly the contrast of television images by reducing the external reflections of the ambient light from said shield.

The X-ray absorbing shield 67 or 76 may also serve as a protection against the television tube implosion hazard and in such case it will be cemented to the surface of the face-plate 66 or 78.

It should be understood that X-ray absorbing shields have uniform or homogenous construction at least in their central portion, which means that they are formed in this region from one and the same material whether it be glass or plastic and do not have any other material intervening in between said material. The term uniform or homogenous construction indicates also that all X-ray shields are in the form of one member as shown in the drawings and which is distinguished from a plurality of rods or fibers united together by fusion or cementing.

The X-ray absorbing shield 67, 67a or 76 may be attached to the television receiver tube by means of a band or cuff 80 of an elastic material such as rubber or Silastic made by Dow-Corning Co. This construction is shown in FIG. 9 in which the X-ray shield 67 is provided around its periphery with elastic extension means 80. The elastic extension member 80 may be provided around the entire perimeter of the X-ray shield 67 or its modifications, or it may be provided only in the limited parts of its perimeter. The elastic member 80 grips tightly the sidewalls of the television tube and holds the X-ray shield in a good contact with the faceplate 66 or 78 of the tube. This construction provided for releasable securing and removing of the X-ray shield which is of advantage in certain applications. It should be understood that this releasable construction applies to all types of X-ray shields and to all television tubes.

Another releasable construction of the X-ray shield is shown in FIG. 10. In this embodiment of invention the sides of the X-ray shield 67 or 76 extend backwards over the sides of the television tube. The extensions 800 which may be an integral part of the X-ray shield itself or may be attached to the shield are provided with springs or other retractable or elastic compression members 77 which can be manually deflected to mount them on the sidewall 81 of the tube. When the retraction is removed, the elastic members 77 grip the sidewalls of the tube and secure the contact of the X-ray shield to the sidewall 81. The screw 77a serves to fasten the spring strip 77 to the extension member 80a. It should be understood that this construction also applies to all types of X-ray shieldes and to all television receiver tubes.

The X-ray shield 67 or 76 may be also attached to the television receiver tube by means of suction cups 84. The suction cups may be afiixed to the X-ray shield 67 or 76 itself or its extensions 80a and may be mounted on the side-walls of the television recevier tube as it is shown in FIG. 11. In cases in which the X-ray shield is supported by the cabinet housing the television set, the suction cups 84 may be attached to the wall of said housing.

It should be understood that the term attaching memher or means used in specification and in the appended claims embraces elastic attaching members, spring-like members, framing members, extension panels such as side-panels or base panel or top-panel and all other mechanical appliances which serve to mount the X-ray shield in its position. It should be understood that the term chemical attaching means embraces the use of all chemicals, plastic materials or soldering glasses which serve to mount the X-ray shield described above in its position. It should be understood that the term suction means embraces all devices which serve to If the reduction of the X-rays emitted from the color television set is accomplished by the shield 67 alone, then shield 67 and its modifications should be of a thickness and composition which will reduce the X-rays emitted from color television tubes described above by a factor of at least. In many cases, especially for protection of children, it is obligatory that said shield 67 should reduce transmitted X-rays by a factor of 12 or more.

It was also found that the X-ray absorbing power of the X-ray shield 67 and its modifications should be equivalent to at least 0.015 mm. of lead or more for the use in color television receivers described above and emitting X-rays of wave-length longer than 0.35 A. In some cases, especially for protection of children, equivalent thickness of at least 0.020 mm. of lead is necessary; but less than 0.35 mm. of lead as it was found that it will impair transparency to colors.

The purpose of this invention may be also accomplished by the modification shown in FIG. 6. In this embodiment of the invention the X-ray shield 67a is mounted on the inside surface of the face-plate '66. All features of the X-ray shield 67 described above apply to the modified X-ray shield 67a. However the glass or plastic material used for shield 67a must be chemically compatible with the fluorescent material of the screen 64. It was found however that this modification results in a loss of contrast of images,

Another modification of this invention is shown in FIG. 7. In this embodiment of invention the X-ray shield 67 or 67a is replaced by a special face-plate 68. The endwall 68 (face-plate) is now used without a shield 67 or its modifications which means that said endwall has an exposed uncovered external surface. The face-plate 68 is light transparent and is made of a thickness and of composition comprising X-ray absorbing elements which will reduce the amount of X-radiation escaping from the television tube 51 to not more than 0.1 mr./ hour. In some cases, especially for children protection, it is obligatory to reduce escaping X-rays to the rate not exceeding 0.040 milliroentgen per hour or less as was explained above. This may be accomplished by increasing the thickneses of the face-plate which is the endwall 68 above the present thickness of 1 cm. to the thickness of at least 1.3 cm. and not more than 2 cm. It was found however that the weight of greatly increased thickness of the faceplate becomes intolerable. In the preferred embodiment of this invention the adequate X-ray protection from the X-rays of wave-length longer than 0.35 A. was accomplished by increasing the proportion of heavy metal oxides such as barium oxide or titanium oxide or lead oxide or cerium oxide. If barium oxide is used the best results were obtained by using it in concentration higher than and lower than The use of lead oxide in high concentration for this purpose comes within the scope of this invention but is difiicult in the color television receivers because of lower melting point of a glass comprising said lead oxide. Regardless of the choice of construction for the face-plate 68, it must provide the X-ray absorbing power such as to reduce the amount of X-rays escaping through it to the rate not exceeding 0.1 mr./hour. In some cases, especially for children protection it is obligatory to reduce escaping X-rays to the rate not exceeding 0.040 millirentgen per hour.

All aforesaid X-ray intensities being measured at the distance of 5 cm. from the endwall 68 of the tube as was explained above.

It was found that the X-ray absorbing power of the faceplate 68 if used alone should be greater than the equivalent X-ray absorbing power of 0.185 millimeter of lead. In some cases, especially for protection of children, it is necessary to provide the X-ray absorbing power greater than 0.190 mm. of lead. The X-ray absorbing power of faceplate 68 or its modifications should not be greater than that of 0.35 mm. of lead, as the transparency to colors will be impaired. Furthermore it should be understood that the face-plate 68 may be completely transparent or may be provided with or be formed by a light partially absorbing material to reduce internal reflections of light.

It should be also understood that aforesaid faceplate may have a matt or etched external surface in order to reduce specular reflections of the external light.

The neutral density filtering efl ect is accomplished by incorporating into the glass or plastic of which the X-ray shield 67, 67a or 76 is formed, small amounts of iron manganese and of cobalt.

It should be understood that all modifications of the invention described above apply as well to the black and white image television receiver tubes. Such television tube 70 is shown in FIG. 8. The tube 70 has only one electron gun 54 and does not have the shadow mask 63 which is used in the color receiver tube. The scanning electron beams 73 produces in the fluorescent screen 74 black and white picture. The X-ray shield 76 may have the same construction as described above for the shield 67 or 6711.

The thickness of the X-ray shield 67 and its modifications and of faceplate 68 will vary in color television tubes considerably according to the type of the X-ray absorbing material used and its percentage and was explained above. Whatever the thickness is selected, the combined X-ray absorbing power of the face-plate 68 and of the X-ray shield 67 or 76 must be such as to reduce the amount of X-rays escaping through them to the rate not exceeding 0.1 mr./hour. In some cases the escape of X-rays must be not more than 0.040 milliroentgen per hour.

This invention applies especially to the color television receiver tubes in which the peak energy wavelength of the emitted X-rays from the fluorescent screen in said receivers is shorter than 0.5 A. but does not exceed 0.35 A. which means that this wavelength is not shorter than 0.35 A.

It should be also understood that this invention applies especially to the television receiver tubes in which the image screens 64 or 74 have a surface larger than 2000 cm. When using small receiver tubes the observer may watch the television screen without being exposed to the direct X-ray beam coming out of this screen. On the other hand when the image screen 64 or 74 is larger than 2000 cm. the child who sits close to the television screen cannot avoid being exposed to the full X-ray beam. It is for this type aof television receivers that the present invention will serve as a protection.

It was found that for color television receivers described above and operating at 27 kv. or higher, the X-ray absorbing power of the face-plate 66 in combination with X-ray shield 67 or its modifications should be at least equal to the equivalent X-ray absorbing power of 0.185 mm. of lead. In some cases it is preferable especially for protection of children, that the X-ray absorbing power of aforesaid combination of the faceplate 66 and of the X-ray shield 67 should be equal to at least 0.190 mm. of lead. In all cases the X-ray absorbing power of the faceplate 68 (endwall) alone or the combined X-ray absorbing power of the endwall 68 and shield 67 should not exceed the equivalent X-ray absorbing power of 0.35 mm. of lead as explained above.

It was found that all color television tubes described above and which operate with high accelerating potentials e.g. above 25 kv. suffer from discoloration of the glass of their endwalls 66 or 68. The reason for said discoloration was found to be due to multi-crystal granular and discontinuous construction of the phosphor screen 64. This allows some electrons of high velocity to pass between the crystals or particles of fluorescent material and impinge on the glass end wall (face-plate) of the tube. This problem was solved by the use of a very thin layer 55 which is transparent to fluorescent light and which is absorbing for said electrons. The layer 55 is mounted on the internal surface on the endwall 66 or 68 as shown in FIGS. 7, 8 and 9 This protective layer 55 mount the X-ray shield in its position by suction. It should be understood that all modifications of means for attachment or mounting of the X-ray shields described above apply to all types of television tubes and to all types of X-ray shields. It should also be understood that the X-ray shields may be mounted outside of the television cabinets or inside of said television cabinets, and that said X-ray shield may be mounted in optical contact with the face-plate of the television receiver tube or spaced apart from said face-plate.

It should be understood that all television receiver tubes described above may be of electrostatic type or of magnetic type or of the combined type. It should be further understod that the face-plates 66, 68 or 78 and X-ray shields 67, 67a or 78 may be of curved shape, of convex shape or of a flat shape.

It should be understod that this invention applies specifically to the television receiver tubes both of black and white and of color type which are used for home and which are operating at voltages below 40 kv.

Although I have shown and described certain specific embodiments of my invention, I am fully aware that many modifications thereof are possible. My invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims.

What I claim is:

1. A vacuum tube for color television comprising means for producing a beam of free electrons, means for accelerating said electrons, means for scanning with said beam of electrons, fluorescent means for receiving said scanning beam of electrons and producing a multi-color image, said fluorescent means mounted on the face-plate of said tube, said fluorescent means when impinged by said electrons emitting visible light and producing a television image, said electrons furthermore when impinging on said fluorescent means and other parts of said tube causing emission of X-rays, said face-plate constructed of light transparent material which is substantially free from lead oxide and is provided with means for absorbing X-rays produced in said tube and reducing said X-rays transmitted through said face-plate of said tube to the amount smaller than 0.04 milliroentgen per hour measured at the distance of 5 cm. from said face-plate, measured with said face-plate having external surface uncovered and with said tube operating at 30 kv. potential and with said beam of electrons being of the standard amperage used in color television tubes, said face-plate having furthermore the thickness of at least 1 cm.

2. A device as defined in claim 1 in which said X-ray absorbing means comprise more than of an oxide of alkali earth metal and less than 20% of said oxide.

3. A device defined in claim 1 in which the main X-ray absorbing power of said X-ray absorbing means is provided by an oxide of alkali earth metal.

4. A device as defined in claim 1 in which said X-ray absorbing means comprise an oxide of alkali earth metal and which comprises a perforated metallic screen member mounted in the path of said electron beam.

5. A device as defined in claim 4 in which said faceplate has the major part of its external surface uncovered.

6. A device as defined in claim 1 in which an electron absorbing member is mounted between the internal surface of said face-plate and said fluorescent means.

7. A device as defined in claim 1 in which said faceplate has the major part of the external surface uncovered.

8. A device as defined in claim 4 in which said X-ray absorbing means comprise more than 10% of an oxide of alkali earth metal and less than 20% of said oxide, and in which said electron beam has about one milliampere at said means for producing said electron beam.

9. A device as defined in claim 1 in which the X-ray absorbing power of said X-ray absorbing means is provided by an oxide of alkali earth metal, and in which said face-plate has curved configuration and the major part of the external surface of said face-plate is uncovered.

10. A device as defined in claim 1 in which said X-ray absorbing means comprise an oxide of alkali earth metal, and in which said X-rays are measured with said beam of electrons having substantially 1 milliampere at said means for producing said electron beam.

11. A device as defined in claim 1 which comprises a perforated metallic screen member mounted in the path of said electron beam.

12. A device as defined in claim 11 in which said beam of electrons has substantially 1 milliampere at said means for producing said electron beam.

13. A device as defined in claim 10 in which the main X-ray absorbing power of said X-ray absorbing means is provided by said oxide, and which comprises a perforated metallic screen member mounted in the path of said electron beam.

14. A device as defined in claim 1 in which said faceplate has X-ray absorbing power higher than the equivalent X-ray absorbing power of 0.190 mm. of lead.

15. A device as defined in claim 1 in which the X-ray absorbing power of said X-ray absorbing means is provided by an oxide of alkali earth metal, in which said X- ray absorbing means comprise more than 10% of said oxide and less than 20% of said oxide and in which said face-plate has X-ray absorbing power higher than the equivalent X-ray absorbing power of 0.190 mm. of lead.

16. A device as defined in claim 14 in which the main X-ray absorbing compound in said X-ray absorbing means is an oxide of alkali earth metal.

17. A device as defined in claim 14 in which said X- rays are measured with said beam of electrons having substantially 1 milliampere at said means for producing said electron beam and which comprises a perforated metallic screen member mounted in the path of said electron beam.

18. A device as defined in claim 14 in which said faceplate has a major part of its external surface uncovered.

References Cited UNITED STATES lPATENTS 2,291,406 7/1942 Paehr 313-64 2,676,109 4/1954 Barnes et al 313-64 X 3,046,148 7/1962 Middleswarth 313-64 X 3,422,298 1/1969 De Gier 313-64 JAMES W. LAWRENCE, Primary Examiner V. L. LA FRANCHI, Assistant Examiner U.S. Cl. X.R. 

